1. Field of the Invention
The present invention relates to a printing relief plate producing apparatus, a printing relief plate producing system, a printing relief plate producing method, and a recording medium for producing a printing relief plate having a plurality of halftone dot convexities for transferring an ink to a print medium to print halftone dots thereon.
2. Description of the Related Art
Heretofore, printing relief plates have been used in flexography, for example. As well known in the art, flexography uses elastic plate materials together with aqueous and UV inks. Since the plate materials are elastic, they lend themselves to printing on corrugated cardboard materials having surface irregularities.
Flexography has been problematic in that, since the used plate materials are elastic, halftone dots that are printed tend to be large in size, resulting in high dot gain and graininess (i.e., density fluctuations indicative of image coarseness).
Japanese Laid-Open Patent Publication No. 2008-230195 discloses a printing relief plate for printing on a can barrel. The disclosed printing relief plate has convexities the height of which is smaller than the height of a solid area of the printing relief plate. According to the publication, convexities that are lower than the solid area are less liable to be deformed when pressed by a blanket, and hence such convexities are effective at preventing dot gain from increasing.
Japanese Laid-Open Patent Publication No. 2008-183888 also discloses a printing relief plate for printing on a can barrel. The disclosed printing relief plate has convexities for printing halftone dots the halftone dot area ratio of which is equal to or smaller than a prescribed value. The height of the convexities becomes lower as the halftone dot area ratio is reduced. According to the publication, convexities for printing halftone dots, the halftone dot area ratio of which is small, bite into a blanket by a reduced distance, thereby reducing enlargement of the small halftone dots.
Japanese Laid-Open Patent Publication No. 2007-185917 discloses a flexographic printing plate including a halftone dot area the height of which is smaller than the height of a solid area of the printing relief plate by 0 μm to 500 μm, at a halftone dot area ratio equal to or greater than 5% and a halftone dot area ratio equal to or smaller than 40% on printed images. According to the publication, it is possible to produce a printing relief plate that exhibits excellent dot gain quality.
Japanese Laid-Open Patent Publication No. 2006-095931 discloses a platemaking method for generally shortening a platemaking time required to produce a printing relief plate for flexography, using laser beams having first and second beam diameters.
However, the printing relief plates disclosed in Japanese Laid-Open Patent Publication No. 2008-230195, Japanese Laid-Open Patent Publication No. 2008-183888, Japanese Laid-Open Patent Publication No. 2007-185917, and Japanese Laid-Open Patent Publication No. 2006-095931 pose certain problems related to engraving accuracy and print reproducibility, if the height (engraving lowering quantity) of the convexities for all of the halftone dots is changed altogether to a certain level at the same halftone dot area ratio.
The first problem is that, since the height of the convexities is constant for a screen tint region within a highlighted area, even a slight error from a target engraving quantity is liable to cause a printing density shift. For achieving stable printing density, therefore, it is necessary to maintain engraving accuracy for a target convexity height.
The second problem is concerned with a halftone dot printing failure. More specifically, as shown in FIG. 22A of the accompanying drawings, an engraved printing plate has a solid area 2 and lowered halftone dot convexities 4, 4a, 4b, which altogether are lower than the solid area 2 by a height hc. In a printing process, almost no printing pressure is applied to the lowered halftone dot convexities 4a, 4b, which are disposed in a boundary region between the solid area 2 and the lowered halftone dot convexities 4, 4a, 4b. Therefore, no halftone dots are printed, or the printed halftone dots become blurred within an area 8 of a print sheet 6, which is positionally aligned with the lowered halftone dot convexities 4a, 4b. 
Consequently, as shown in FIG. 22B of the accompanying drawings, a resultant print 6p includes an area 8p where no halftone dots are printed, or where the printed halftone dots become blurred, within a halftone dot area 4p near to the solid area 2p shown in crosshatching.
The third problem is that, inasmuch as during the printing process, printing pressure is applied unstably to adjacent halftone dot areas having different halftone dot area ratios, different printed areas tend to exhibit different printing densities. As a result, print reproducibility becomes unstable when prints are repeatedly produced.